The present disclosure is generally directed to phase change inks, Moreover, the present disclosure is directed to the preparation of phase change inks using dry flushed pigments. In an acoustic or piezoelectric inkjet system, ink droplets are propelled to the recording medium by means of a piezoelectric oscillator. In such a system, a recording signal is applied to a recording head containing the piezoelectric oscillator, causing droplets of the ink to be generated and subsequently expelled through the printhead in response to the recording signal to generate an image on the recording medium. In this printing system, a recording signal is converted into a pulse by a signal processing means, such as a pulse converter, and then applied to the piezoelectric oscillator. A change in pressure on the ink within an ink chamber in the printhead caused by the recording signal results in droplets of ink being ejected through an orifice to a recording medium. Such an inkjet system is described in more detail, for example, in U.S. Pat. No. 4,627,875, the disclosure of which is totally incorporated herein by reference.
Thermal inkjet printing processes are described in more detail, for example, in U.S. Pat. Nos. 5,169,437 and 5,207,824, the disclosures of which are totally incorporated herein by reference.
In these and other inkjet recording processes, it is necessary that the ink being used meet various stringent performance characteristics. These performance characteristics are generally more stringent than those for other liquid ink applications, such as for writing instruments (a fountain pen, felt pen, and the like).
Further, there are many requirements for the liquid compositions including the above-described inks for inkjet recording, and specific examples thereof include. (1) no clogging of nozzles of inkjet recording heads; (2) superior ejection stability and frequency responsiveness; (3) good recovery of smooth ink ejection after residing in printhead for a long time, such as greater than two weeks; (4) no generation of precipitates even after long-term storage; (5) no corrosion-deterioration of members, such as the recording heads, which contact therewith; (6) provision of favorable printing quality; (7) safety and no unpleasant odor; and the like.
Various inks for inkjet printing processes are known in the art. For example, various inkjet inks are disclosed in U.S. Pat. Nos. 4,737,190 and 5,156,675.
Although numerous inkjet inks are presently available, they generally do not meet all of the above-described requirements, while also providing excellent print quality on plain paper. In particular, the inks generally used in inkjet printing processes, while producing acceptable print quality, do not produce the high print quality that is achieved by using dry toner compositions, such as in electrostatographic imaging processes.
A need continues to exist in the inkjet industry for improved inkjet inks, and processes for producing the same, that satisfy the above-described requirements while providing high quality prints on a wide variety of recording media, including plain paper. Although some currently available inkjet inks may provide waterfast images with better substrate latitude, the inks are unacceptable in that they generally smear and have poor latency and maintainability characteristics. In addition, such inks are generally difficult to manufacture. Thus, there remains a need in the inkjet ink industry for improved black and colored inks that can be easily prepared and can be obtained at a lower cost.
One type of inkjet ink is a phase change ink that contains pigments. Pigments are a lower cost alternative to dyes. Pigments are insoluble, fine particle size materials used in a number of applications including ink formulations, coatings, paints and the like to provide color, to hide substrates, to modify the properties of coatings, and to modify the performance properties of films.
However, pigments are often supplied by the manufacturer as dry aggregates and agglomerates that are many times larger than the primary particle size (which often is less than 100 nm in diameter). Thus, with these pigments, a pigment manufacturer must perform a size reduction step. For example, the pigments needs to be reduced to a size nominally about 100 nm in diameter, with a narrow particle size distribution in the final ink formulation. The size reduction is necessary in order to achieve good ink jetting and print quality performance. Print head jet sizes are getting smaller and smaller, so a small pigment particle size is crucial in high quality inks. Pigments useful for pigmented phase change ink application range in cost from less than $20 per kilogram to less than $100 per kilogram. However, the particle size reduction costs also need to be added into this preparation cost.
The technology utilized in pigment particle size reduction is an important field in the printing and coating industries. In xerographic printing applications, aqueous pigment dispersions for Xerox's emulsion aggregation toners are produced by milling the solid pigments together with a surfactant and water in milling equipment such as, for example, small media mills, homogenizers and the like. This results in pigment particles sizes ranging between 100 and 200 nm in diameter. Similar pigments used in paints and other coatings are size reduced by means of various media mills technologies. For example, in the case of conventional toners for monochrome xerography, carbon black is dispersed in the toner medium by means of extrusion melt mixing.
An effective known method of size reducing pigments is the formation of pigment concentrates or masterbatches. The pigment concentrate is then milled down to the final pigment loading for the product, where the pigment loading may be as low as 1 to 2 percent by weight. Under certain operating conditions, the pigment concentrate is produced in one location and the final product in another location that may be hundreds or thousands of kilometers away. Therefore the cost of transportation is an important factor in the economics of making pigmented products.
In the case of pigment phase change ink, it is useful to produce a pigment concentrate including pigment and a carrier (for example, wax such as Kemamide® S-180 stearyl stearamide wax). Initially, the concentrate was made using an attritor and this produced concentrates comprising about 20 percent by weight of pigment. This process was performed at elevated temperatures (about 120° C.) for periods up to seven days. Pigment concentrates were then made using a more efficient process, where the concentrate was produced in a basket or immersion mill (for example, the Hockmeyer immersion mill). The Hockmeyer micro mill included a jacketed vessel and a milling head (basket assembly). The grinding media utilized in the milling head can include for example, zirconia particles. The milling time for pigment concentrates containing pigments as high as 40 percent by weight has been reduced to less than 5 hours. Thus, the cost of making a pigment concentrate in a process that requires 5 hours is substantially less than in a process that requires 7 days.
Although the Hockmeyer process provides good quality dispersion in the carrier within a few hours, the Hockmeyer process requires operation of the disperser blade in the milling head at very high rotational speeds, for example, 5000 revolutions or more per minute or higher. This high energy operation results in excessive wear of not only the milling media, but also the parts of the milling head (that is the shaft, peg hub, counterpegs, screen, etc.). This in turn results in contamination of the pigment concentrate and the need for frequent parts replacement. A further significant issue is that there is a need for a wetting step in the pigment dispersion process. The dry pigment has to be wetted in the carrier for periods of time equal to or greater than the milling time. Accordingly, there is a need to further reduce the cost of producing the pigment concentrates for the phase change ink formulations.
An effective method for supplying finely dispersed pigments for toners for xerographic printing applications is via flushed pigments. Flushed pigments have significantly better pigment dispersion in melt mixed toners versus dry pigment. In a nominal pigment manufacturing process, pigments can be precipitated or crystallized out of an aqueous mixture or solution. After the water is filtered out, the product is called presscake (or wetcake). If the presscake is further dried to produce pigment powder, the aggregated and/or agglomerated pigment particles require high amounts of energy to redisperse in a toner in a melt mixing device (for example, an extruder). In the flushing process, the presscake is mixed together with the toner resin at high pigment loading and heated in a flusher (for example, an extruder, sigma mixer or the like). As the toner resin heats up and then softens with an optional application of vacuum, the water between the pigment particles is displaced by the resin. This results in a mixture of pigment in resin which is called “flushed pigment” or “wet flushed pigment.” The name “wet flushed pigment” is utilized (that is, “flushed pigment” or “flushed pigment from presscake”) to differentiate from the dry flushed pigment of the present invention. The advantage of the flushing processes is that the pigment particles are not allowed to aggregate, thereby the dispersion quality of the flushed pigment in toner is better than for dry pigment. For example, U.S. Pat. No. 5,866,288 describes the use of flushed pigments for toners and the better dispersion quality.
US Patent Application 20090297714, assigned to Xerox and hereby incorporated by reference, discloses a process for the preparation of phase change inks where pigments are introduced to the ink formulation in the form of flushed pigments. The flushing process includes aqueous presscake of pigments converted to flushed pigment with the addition of various ink components such as wax, etc. However, there was no identification as to whether this process performed sufficiently to produce high quality inks.